What does the Fixed Frame mean in rviz?

edit

Thank you so much.

Does the Grid always be the world coordinate? If I set /odom as the fixed frame. Rviz will draw obstacle at potions (10, 5) in the Grid?

I think the heading in your assumption should be pi/2.

Yeah, fixed that, sorry. Need coffee.

And yes, if you set the fixed frame to /odom, the obstacle will be drawn at (10, 5).

@tom, I didn;t really follow your explanation, what does heading = pi/2 mean ( is the robot facing left by 90 degree compared to lidar) ?

also could you elaborate how the (10,0) and (10,5) works out ? can't seem to visualize that based on the explanation.

What happens If I keep my fixed_frame as /map instead of /odom and /base_link frame?

Then you will see everything displayed in the map frame. In my drawing, just imagine another coordinate frame like the odom frame, but not with the same origin/position.

All the fixed frame does is define where the origin is when rviz displays things.